The present invention relates to a method for confirming the reliability of a blood alcohol concentration value to be measured from respiratory air. In this method, the incoming exhalation air is sampled during exhalation, for at least one measured alcohol concentration value and, during the same respiratory stage, it is sampled for at least one measured carbon dioxide concentration value. A result is produced which is proportional to the blood alcohol concentration value and which is based on one or more measured alcohol concentration values obtained during exhalation from the lungs.
The invention relates also to an apparatus for implementing such measurement, said apparatus comprising sensor elements for obtaining a measured alcohol concentration and carbon dioxide concentration value from the incoming exhalation air stream as well as output elements for producing, if necessary, a result which is proportional at least to the blood alcohol concentration.
For quite some time, the detection of blood alcohol content has been effected by means of testing devices which measure the air stream exhaled by a subject for its alcohol concentration which, as known, is to a certain degree proportional to the blood alcohol content, provided that the measured exhalation air originates in the deep lungs, and thus consists of so-called alveolar gases. Hence, the measurements are based on the hypothesis that a given alcohol concentration value measured from exhalation air always corresponds to a given blood alcohol concentration value. However, an effort to determine blood alcohol concentration by means of the alcohol concentration in exhalation air involves several sources of error. The publication DE 2,928,433 pursues a solution to the problem that the alcohol concentration of exhalation air fluctuates in time with the heart rate, which of course is not the case with the concentration of blood alcohol. As a solution to this problem, the cited publication discloses a control device capable of logical functions and calculation. On the other hand, the publication U.S. Pat. No. 5,376,555 describes an arrangement for eliminating the effect of possible, so-called mouth alcohol at the initial stage of sampling the respiratory air. The fact is, namely, that if alcohol has been ingested just prior to measuring, the alcohol contained in the mouth as a result thereof produces a relatively high concentration peak in the alcohol content measured at the start of a breath sample. The effect of this peak is eliminated as described in the cited publication by making use of the carbon dioxide concentration also measured at the early stage of exhalation. If this is not done, the result may be a high breath alcohol concentration and, on the basis of presumed correlation, a too high estimate for blood alcohol content that does not correspond to true blood alcohol content. Thus, an object of the cited publication is to eliminate the incorrectly excessive alcohol concentration caused by mouth alcohol.
In addition to the above sources of error, there are other sources of error which produce too low an alcohol concentration measure with respect to true blood alcohol content. For example, if a subject takes a few or several very deep breaths to create a hyperventilation prior to measuring alcohol concentration from alveolar air, the alcohol concentration measure obtained thereafter will be lower than it would be had the subject breathed in a normal manner. As a result of this, the estimate of blood alcohol content made on the basis of the measured value is also too low. This is the case even if the air exhaled by a subject in actual alcohol measuring has a sufficient volume and comes from the deep lungs in a proper manner and, thus, consists of alveolar gases. This hyperventilation has been described e.g. in the book Z. Kalenda: MASTERING INFRARED CAPNOGRAPHY, 1989.
An incorrect result is also obtained if a subject, during measuring, restricts the amount and/or duration of his or her exhalation. Also in this case the measured alcohol concentration will be lower than what it would be had the subject exhaled from the deep lungs in a normal manner and also the estimate of blood alcohol content made on the basis of the measured value will be too low.
A solution to this latter problem has been pursued e.g. by training the measuring device operating personnel, whereby the measuring device operator aims to oversee that a subject being examined exhales properly into the measuring device. However, this procedure is highly unreliable and different persons have substantially different pulmonary capacities and, thus, there are no guarantees regarding a sufficient exhalation time and/or exhalation volume.
There are also situations which can produce an excessive measured alcohol concentration value. Such a situation is for example hypoventilation which is a reverse situation to the above hyperventilation and in which a subject breaths less than normal. In a hypoventilation situation, the CO.sub.2 concentration and alcohol concentration of an alveolar gas are higher than which would be the values correctly corresponding to the concentrations in blood. Thus, the alcohol concentration determined from an alveolar gas in a hypoventilation situation is higher than the equilibrium alcohol content of the human body and blood. In hypoventilation, a subject can be falsely convicted of intoxicated driving if the hypoventilation is not detected.
The publication U.S. Pat. No. 3,830,630 discloses a system, wherein a resistance bridge consisting of filaments is used for measuring both CO.sub.2 -content and alcohol content from exhalation air. These two content measurements are linked to each other such that, if the measured carbon dioxide content rises to a minimum value of 4.5%, the resulting alcohol measurement is found to be correct. The cited publication states further that the CO.sub.2 -content and alcohol content are in equilibrium with blood alcohol content when the carbon dioxide content is 5%-5.25%. As for the above-described error situations, this prior art system only eliminates those caused by hyperventilation and even that requires that it indeed be alveolar exhalation air which is being measured. The publication mentions nothing about the necessity of monitoring this, nor does it describe any means for securing this aspect. This system may cause further errors for the reason that various persons have individual differences in the carbon dioxide concentration of a normal alveolar gas, the fluctuation range being roughly 4.7%-5.5%. If, for example, the normal alveolar CO.sub.2 -content of a person is 5.5% and the person blows into an alcohol measuring device and stops exhalation before the exhalation air comes from the sufficiently deep lungs, the exhalation may have a maximum CO.sub.2 -content of for example 4.6%. According to the cited publication, this result is acceptable. As a matter of fact, the alcohol content measured from this particular exhalation is too low.